Compare Maps of Prairie Landforms and Sediments near Medicine Lake, Montana
How to compare the USGS Medicine Lake topo map, MBMG Report 738 and MBMG 540 to read glacial landforms and sediments while keeping buried geology separate.
Contents
The short answer
Around Medicine Lake in Sheridan County, no single map tells the whole story. Three sources work together. The 2024 USGS Medicine Lake quadrangle shows the shape of the land surface. Montana Bureau of Mines and Geology (MBMG) Open-File Report 738 explains what sediments make up that surface and what lies buried beneath it. The older MBMG 540 surficial-geology map adds regional sediment units, but only north of 48.5°N. Use the topo map for shape and location, Report 738 for local sediment and subsurface interpretation, and MBMG 540 only where its coverage actually reaches. Keep three kinds of evidence apart: visible landforms, mapped surface sediments, and buried aquifer deposits.
What each map is built to do
| Map | Coverage and scale | What it shows | Dates to note | Best use |
| USGS Medicine Lake quadrangle (2024) | 48.375–48.5°N, 104.5–104.375°W; 1:24,000 | 10-foot contours, roads, names, imagery-based base map; UTM zone 13U, NAVD88 elevations | Roads 2016, imagery 2017, elevation data 2009 | Land-surface shape, local anchors, measuring distance |
| MBMG Report 738 text and figures (April 2021) | Clear Lake aquifer area and surroundings | Sediment types, buried valleys, borehole-based cross sections; Figure 8 separates till, meltwater sand and gravel, lake silt and clay, stream alluvium and windblown sand hills | 2021 report | Interpreting what a landform is made of and what lies beneath it |
| Report 738 Plate 1 | Aquifer study area | Monitoring and shoreline wells, aquifer and model boundaries, coordinates, distance bars | 2021 | Locating wells and study boundaries |
| Report 738 Plate 2 | Regional | Physiography and drainage: Medicine Lake and Clear Lake swales, diversions, glacial limits, approximate buried-deposit boundaries, possible buried terrace scarps | 2021 | Regional glacial and drainage context |
| Report 738 Plate 3 | Selected wells | Hydrographs tied to outwash and alluvial aquifer categories and management areas | 2021 | Groundwater behavior, not land shape |
| MBMG 540 | 1:100,000; southern edge at 48.5°N | Preliminary surficial geology: alluvium, pond deposits, eolium, outwash, ice-contact deposits, glacial lake deposits, till | Mapped 1947–1980, published 2006 | Regional surface-sediment context north of 48.5°N |
The differences that matter when you compare them
Coverage is the first check. The quadrangle's northern edge and MBMG 540's southern edge both sit at 48.5°N, so the two maps meet along a line rather than overlapping. MBMG 540 cannot tell you what sediment lies within the quadrangle, and its units should not be extended southward across the lake. For points inside the quadrangle, Report 738 is the geology source that applies.
Surface versus subsurface is the second. Contours on the quadrangle show land-surface elevation. Plate 3 shows groundwater elevations in wells, which are a different quantity and should never be read as terrain. Report 738's central finding concerns buried geology: the Clear Lake aquifer is interpreted as ancestral Missouri River alluvium overlain by glacial outwash, filling buried valleys. Those valleys are not necessarily visible at the surface, so a broad swale on the topo map hints at buried channels without proving their exact edges.
Method and confidence is the third. Report 738's surface sediment mapping was drawn largely from soil parent materials with limited field checking, and its borehole cross sections show substantial variation underground. Plate 2 marks some contacts as approximate and some terrace scarps as possible. MBMG 540 is labeled preliminary and reflects field mapping from 1947–1980. Neither geology source is clearly more reliable than the other at a given spot. They mostly cover different ground, and both carry stated uncertainty. Where a decision depends on what is actually underground, the borehole cross sections and the well data archived in the state's Ground Water Information Center (GWIC) carry more weight than either surface map.
Scale is the fourth. At the stated print scale, 1 centimeter equals 240 meters on the 1:24,000 quadrangle and 1 kilometer on the 1:100,000 MBMG 540 map. A boundary line that looks sharp at 1:100,000 can represent a band hundreds of meters wide on the ground. If you view a map resized on screen or reprinted, recalculate from the distance bar rather than the scale ratio.
Reading the sediment units
- Till: poorly sorted debris, from clay to boulders, deposited directly by glacial ice. It usually underlies gently rolling uplands.
- Outwash: sand and gravel sorted and laid down by meltwater streams flowing from the ice. In this area it overlies the older Missouri alluvium in buried valleys.
- Ice-contact deposits: sediment laid down against or on top of stagnant ice. These deposits often have hummocky, irregular surfaces.
- Glacial lake deposits: fine silt and clay that settled in standing water impounded by ice or sediment.
- Alluvium: sediment deposited by streams, either modern or ancestral.
- Pond deposits: fine sediment accumulated in small closed basins.
- Eolium (eolian or windblown sand): wind-deposited sediment. Report 738 maps sand hills of this type southeast of Medicine Lake.
Kettles deserve special attention. Report 738 interprets many of the region's wetland depressions as kettles, which are basins left behind when blocks of stagnant glacial ice melted out of the sediment around them. On the quadrangle, look for small closed contour loops or water symbols. Then check the geology figures for the sediment surrounding them. This is one of the most direct links between glacial history and the prairie wetland basins that observers visit. The other is the contrast between those basins and the sandy uplands mapped as windblown deposits.
A step-by-step comparison workflow
- 1. Confirm the location: Montana, Sheridan County, and coordinates in decimal degrees.
- 2. For each map or figure, record its boundary, scale, legend, projection or grid, and its publication and source dates. The quadrangle's 2024 date is not the date its roads, imagery or elevation data were collected.
- 3. Check whether your point falls inside each map's coverage before reading anything from it.
- 4. Pick one common anchor visible on both the quadrangle and the Report 738 figures or plates, such as a road intersection, a named lake or a well on Plate 1. Treat road and place labels as location aids only, not as evidence of sediment.
- 5. Describe the surface shape from the contours: closed depression, broad swale, hummocky ground or smooth upland.
- 6. Separately, note the sediment symbol at that point and read its definition in that map's own legend.
- 7. Check the cross sections and Plate 2 for buried valleys, approximate boundaries or possible buried scarps near the point. Record these as subsurface interpretation, distinct from what you can see.
- 8. Write down the uncertainty attached to each piece of evidence.
A worksheet with one row per point keeps the evidence separated:
coordinate | map/figure | coverage confirmed | surface elevation/shape | mapped sediment symbol | legend meaning | buried cross-section evidence | date | uncertaintyIllustrative example
This is a hypothetical case to show the reasoning, not a description of a real site. Suppose a point lies at 48.45°N within the Medicine Lake quadrangle, and the contours show a small closed depression. Because 48.45°N is south of MBMG 540's southern edge, that map does not apply, so you turn to Report 738's Figure 8 for the surface sediment. If the figure shows till or ice-contact sediment around the basin, a kettle origin is consistent with the report's regional interpretation. If Plate 2 places an approximate buried-deposit boundary nearby, record it in the buried-evidence column. It does not change the surface landform, and its position is approximate. A road name next to the basin helps you find the spot again, but it tells you nothing about the sediment.
Why this matters for prairie habitat observation
The landforms that shape where water collects near Medicine Lake are mostly glacial. The maps also explain the contrast between wetland basins and windblown sandy ground. Reading the topo map for shape and the geology for material lets you describe a site's setting accurately in field notes, for example as a kettle depression in till or a basin near the sand hills southeast of the lake. The maps describe the physical setting only. They say nothing about current water levels, vegetation, access or what birds are present on a given day.
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